1. Field of the Invention
The present invention relates to thermal printers wherein the selective energization of heating elements causes the transfer of dye to a receiver member.
2.Description of the Prior Art
Some thermal printer apparatus use a dye transfer process. In this process, a carrier containing a dye is disposed between a receiver, such as paper, and a print head formed of for example a plurality of individual thermal heat producing elements often referred to as heating elements. The receiver and carrier are generally moved relative to the print head which is fixed. When a particular heating element is energized, it is heated and causes dye to transfer (e.g. by sublimation) from the carrier to an image pixel in the receiver. The density, or darkness, of the printed dye is a function of the temperature of the heating element and the time the carrier is heated. In other words, the heat delivered from the heating element to the carrier causes dye to transfer to an image pixel of a receiver. The amount of dye is directly related to the amount of heat transferred to the carrier.
Thermal dye transfer printer apparatus offer the advantage of true "continuous tone" dye density transfer. By varying the heat applied by each heating element to the carrier, a variable dye density image pixel is formed in the receiver.
Typically, the print head is organized into a plurality of groups of heating elements. The heating elements of each group are simultaneously addressed in parallel in a pulse width modulation scheme. In this disclosure, when the term addressed is used, it means that a heating element is capable of being energized. The term enabled means that an addressed heating element is energized. When a heating element is addressed, the time that a heating element is enabled will determine the grey scale of an image pixel. The reason heating elements are addressed in groups is that if all the heating elements were energized at the same time, a large and expensive power supply would be needed. When a group of heating elements is addressed, individual heating elements of the group are selectively energized with constant current pulses. The pulse width of a constant current pulse causes its image pixel to have a desired grey scale. When a group of elements is being addressed or undergoing a heating cycle, all other groups are either cold or cooling. After a group of elements has completed its heating cycle, the next group starts its heating cycle. A heating element in the middle of group that is being addressed and energized generally has neighboring heating elements on both sides that are warm. Accordingly, the temperature profile of the interpixel gap between adjacent heating elements, tends to average to some level. Also, over time, the temperature of the print head tends to equilibrate to an average temperature. The temperature of a heating element on the end of a group can however be significantly reduced due to the heat flow to the cold heating element of the adjacent group which is not being addressed and energized. When dye images are transferred with such a printer, low density streaks, or "group lines" can often appear due to the thermal gradient caused by the temperature difference between heating elements of adjacent groups. Also, as noted above, the temperature of the print head itself tends to equilibrate to some average temperature. This average temperature can itself create another problem in that during operation if this average temperature becomes too high, then images often become too dark. In other words, too much dye is transferred to each image pixel. The reason for this is that the average temperature of the heating elements in the print head can increase to a point where it can cause a noticeable amount of additional dye to be transferred.
In order to solve the problem of the average temperature of the print head being too high, the duration of the address signals have been reduced, but the frequency of these address signals is unchanged. The maximum pulse width of constant current pulses that can be provided is reduced. This process effectively reduces the number of density (grey) levels since the maximum time a heating element can be energized is limited by the address signal width rather than the data from the microcomputer.
In another approach to solve this problem, just as in the above-discussed approach, the duration of the address signals are reduced but the frequency of these address signals are maintained at a constant. The difference in this approach is that the frequency of the enable signals is increased so there will be the same number of grey levels available. Viewed differently, the pulse width of the address pulses is adjusted. The maximum pulse width of the constant current pulse that can be provided is reduced but the maximum number of pulse width levels is maintained. This method requires complex circuitry. The modulation data must be processed at a higher speed. The thermal print head must also be able to operate at a higher frequency.
It should be noted that with both these arrangements, the group line effect problem is still not solved.